When Zeno first met Yona, he did not receive the initiation like the other three dragons. It is later on revealed that he is the first generation Ouryuu, thus he wouldn't receive the initiation due to his previous loyalty to King Hiryuu. Throughout the series, he's also seen to not pay any special attention to Yona, while the others did. He treats her just like a normal comrade of his and never called her by her name, usually by "Miss". However, he seems to find Yona extremely cute and will be her "shield" if need be.
Later on, it is revealed by Zeno himself that he has been following Yona around ever since she left the castle. He knows that Yona is the reincarnation of King Hiryuu and has been watching her gather the dragons and overcoming obstacles. He stated it was because he wanted to know if Yona was capable/worthy of using the power of the four dragons. After watching her leave Awa, he thought it was alright to entrust himself to Yona like he did for King Hiryuu and went out to meet them after that.
Like with the others, Zeno also treats Hak as a fellow companion. Zeno considers Hak as a man who's incredibly strong, claiming that his power is not normal for a human. Zeno also seems to have an understanding of Hak, Yona, And Soo-won's complex past. He hardly speaks on the matter though, often times just watching or listening with a grave face. He has however expressed his concerns about the matter with Hak once or twice. When Hak wakes up from a nightmare on the subject, Zeno warns Hak that his blood thirst is apparent. He tells Hak to be careful and to take better care of himself, as the dragons can be reincarnated, but Hak can not.  Hak comments back that "you guys" also can't be replaced. Which in turn makes Zeno express a large, genuine smile, and he sincerely thanks Hak for saying so. It is generally unknown what Zeno's full thoughts are on the situation, but he genuinely cares for Hak and wants him to care more about himself.
Zeno first joined the group because of Yoon's superb cooking; according to him, he's "attracted by the smell of tasty food". Later this was revealed to be an excuse he used to meet Yona without rest of them getting suspicious. He also claims Yoon to be a rather scary boy and later starts to respect him. He also shows concern for Yoon and has the habit of calling him "kid" (sometimes also translated as "lad"). Yoon always reminds Zeno to act his age after they find out that Zeno is actually 2000 years old when he acts younger then them. From some of the comic cover, we can see that both of them are close. Yoon also cares about Zeno whenever he gets injured by saying "I don't want to wash your clothes covered in blood anymore," at the water tribe Territory Ryusui.
Shin-Ah is probably the closest to Zeno aside from all other dragons as they are always seen together (mostly because Zeno loves to snuggle in Shin-Ah's fluff) and Shin-Ah seems to feel calm around Zeno. Like the rest of the group, Zeno is heavily protective of him and seems to show a subtle older-sibling-like fondness for him. He continues his habit of nicknaming all of his companions with Shin-Ah, as he only ever refers to him as 'Seiryuu'.
Jae-Ha cares for Zeno like how he does for the others. After Zeno reveals his real identity, Jae-Ha uses this as a means of annoying him as Zeno dislikes being teased for his age. Even though Zeno is capable of getting himself out of danger without any harm due to his regeneration abilities, Jae-Ha still protects him as he is supposed as the "older brother of the dragons" (if you ignore Zeno's actual age). This of course causes banter between the two, with Zeno often being very annoyed with Jae-Ha's protective tendencies. Zeno gets especially annoyed when Kija and Jae-Ha planned a diversion tactic to prevent him and Shin-Ah from revealing their destructive powers in the Unveiling of the Four Dragons Tournament.
Kija is shown to have a complicated relationship with Zeno. He originally accused Zeno of not caring for his body or honing his powers, and allowing them to fall to the wayside. That considered, he was still moved to tears when he thought about the fact that the four dragons were reunited for the first time in several millenia. He often gets angry with Zeno's childish personality, but cares for him like a brother. When Zeno's backstory is revealed, Kija becomes very mindful of his age and worries about his "old body". Kija laments that he seems to be the only dragon with a happy upbringing and tells Zeno that, if he were ever lonely, he would have always been welcome to come and stay in Hakuryuu village. It was also mentioned in the manga series that Zeno had in fact visited Kija as a child when he was bathing in the early morning.
Zeno knew Tae-Jun's identity right when he first met him despite his disguise which worried Tae-Jun for a while. Tae-Jun said that Zeno, despite his looks, could actually spout some really wise things. Later on, Zeno predicted that Tae-Jun will be a really important person to the Fire Tribe and is seemingly surprised when Tae-Jun reveals that he does not desire to be somebody important for the tribe.
Being sworn to protect Hiryuu through Zeno's dragon's blood, Zeno respects King Hiryuu deeply and calls him "Lord Hiryuu". To him, Hiryuu was the one who brought all the dragons together and made them feel actually connected to each other whenever he was around.
When Hiryuu found out about Zeno's powers, he seemed upset and gave him his pendant as proof that he will be with Zeno, even when everyone is gone. He treasures the pendant that Hiryuu gave him a lot, so much so he was afraid that Kaya had accidentally thrown it away when she's cleaning his wounds and even in the present, he still picks it up from the ground after it was cut off.
Zeno saw Guen as a wild boar that acts recklessly and deeply regrets not visiting the Hakuryuu village after they were all separated, even though Guen said that he'll wait for the day Zeno comes to visit him even if he's an old man then. Thus, when Zeno realized that Guen had passed away, he was extremely upset and screamed, begging King Hiryuu, and his dragon brothers not to leave him alone.
Zeno also remarked that Guen and Kija are similar to each other.
He viewed Abi as one of his dragon brothers and was very upset when Abi passed away. He was also the one who protected Abi on the battlefield when his dragon eyes backfired on him and he was paralyzed.
He viewed Shu-Ten as one of his dragon brothers and a guy who is strong and reliable even though the latter doesn't seem to recognize Zeno as a dragon warrior. Zeno was very upset when Shu-Ten passed away.
Kaya was Zeno's wife.
Zeno first meets Kaya when she rescues him from the rain and cleans him up. Later Kaya tells Zeno that she has an incurable disease, which is why she lives secluded from people to prevent them from getting infected as well. He then offers to take care of her for a period of time until she recovers as a favor to her for rescuing him.
Zeno slowly falls in love with Kaya as they continue to live together. However, Kaya's condition gets worse and she couldn't leave her bed because she was too ill, so she asks Zeno to leave before he gets infected as well. However, Zeno reminds her that he won't get sick and carries on to propose to her which touches Kaya and she cried tears of joy. Unfortunately, in the end, Kaya couldn't make it through and dies holding Zeno's hands. Her last words to him were "May we meet in Heaven again", to which the latter nods gently, upset. He stays with her until she is no longer a corpse but a skeleton, hinting that he wouldn't leave her even after her death.
Even after her death, Kaya's impact on Zeno is great. She helped him heal from the death of the other dragons and even taught him to speak in the third-person speech to prevent himself from forgetting his name, to which he later laughed and commented, "I feel just like a kid." She was also why he started to treasure food and the people around him, as living with her made him realize how hard life could be and made him more appreciative. Putting his hands up in the air when he's happy is also a tic he copied from her. He even wears a scarf in a similar fashion to how she wore her's. Even when he told the dragons he was once married with a very casual, light-hearted tone, Zeno misses her greatly as she had a big role in his life.
- Akatsuki no Yona Manga: Chapter 42, Page 19
- Akatsuki no Yona Manga: Chapter 101, Page 12
- Akatsuki no Yona Manga: Chapter 62, Page 26
- Akatsuki no Yona Manga: Chapter 43, Page 5
- Akatsuki no Yona Manga: Chapter 54, Pages 29-30
- Akatsuki no Yona Manga: Chapter 59, Page 14
- Akatsuki no Yona Manga: Chapter 59, Page 15